1. Field of the Invention
The present invention relates to an optical displacement measuring instrument that moves an objective lens or a focusing lens so as to bring a focal point into coincidence with a measuring face, for measuring a profile of the measuring face based on a movement of the objective lens or the focusing lens.
2. Description of Related Art
Conventionally, an arrangement for irradiating light onto a measuring face and measuring a profile of the measuring face has been known (for instance, see a document 1: JP-A-8-128806).
According to the arrangement disclosed in the document 1, a space filter is disposed in an optical path between a signal detecting optical system and a beam splitter. Adjacent to the center of the space filter, a shield is provided. By guiding reflected light from a surface of an object to the shield, a light volume of the reflected light near its optical axis is shielded or reduced, so that a ratio of the maximum value to the minimum value of a light volume of received light detected by the signal detecting optical system is decreased, thereby preventing a decrease in SN ratio.
However, when a profile of a target object exemplarily includes such a curved surface that a slanting angle of its measuring face is larger than a convergent angle of convergent light converged by a lens and incident on the measuring face, the arrangement according to the above-described document 1 may not favorably measure the profile of the target object.
Specifically, according to the arrangement of the document 1, as shown in FIG. 9, convergent light L902 generated by converging parallel light L901 with an objective lens 800 and incident on a target object 900 is reflected on a measuring face 901, thereby forming reflected light L903. When a slanting angle θ11 of the measuring face 901 is equal to a convergent angle θ21 of the convergent light L902, only an outer edge of the convergent light L902 (hereinafter called as convergent-light outer edge) and an outer edge of the reflected light L903 (hereinafter called as reflected-light outer edge) are overlapped while the convergent light L902 and the reflected light L903 are not overlapped at any other portion. In other words, an angle formed between the convergent-light outer edge L902A and the reflected-light outer edge L903A is 0 degree. At this time, the reflected-light outer edge L903A overlapped with the convergent light L902 proceeds along an outer edge L901A of the parallel light L901 (hereinafter called as parallel-light outer edge), so that the reflected-light outer edge L903A cannot be used for detecting a focusing point of a received beam of light.
In addition, as shown in FIG. 10, when the slanting angle θ12 of the measuring face 911 of the target object 910 is larger than the convergent angle θ21 of the convergent light L902, the convergent light L902 and the reflected light L911 are not overlapped with each other. In other words, the angle formed between the convergent-light outer edge L902A and the reflected-light outer edge L911A is less than 0 degree. At this time, the reflected-light outer edge L911A passes through the objective lens 800 and subsequently proceeds at a position external to the parallel-light outer edge L901A, so that the reflected-light outer edge L911A cannot be used as the received beam of light.
Therefore, the angle formed between the irradiating beam and the received beam of light is 0 degree in such an arrangement as shown in FIG. 9 while no received beam of light is present in such an arrangement as shown in FIG. 10. Thus, sensitivity for detecting a focusing point is lost.
When the sensitivity for detecting a focusing point is lost and a profile of the target object in which a curved surface is included (e.g., spherical or columnar object) is attempted to be followed, positioning of a focusing lens may be erroneously controlled so that an extension of a measuring beam constantly penetrates the curvature center of the measuring face, at portions where the slanting angle of the measuring face is larger than the convergent angle of the convergent light. Thus, in a profile measurement record, record corresponding to the portions where the slanting angle of the measuring face is larger than the convergent angle of the convergent light may erroneously show that the portions are obliquely bulged or recessed from the measuring face, so that only useless record may be obtained.
In view of the above, the applicant has previously proposed an optical displacement measuring instrument for improvement (see a document 2: Japanese Patent Application No. 2007-332296).
The arrangement according to the document 2 includes: a light splitter for splitting reflected light having reflected on the measuring face and having passed through the objective lens into two split light of first reflected light and second reflected light; a first light-receiving element array and a second light-receiving element array respectively disposed anterior to a focal point of the first reflected light and posterior to a focal point of the second reflected light, a plurality of pixels being two-dimensionally aligned in each of the first light-receiving element array and the second light-receiving element array; a light-receiving signal operator that, among light-receiving signals from a plurality of pixels within an area predetermined in each of the first light-receiving element array and the second light-receiving element array, excepts light-receiving signals from pixels having the highest brightness to the nth highest brightness (n being an integer) and obtains a total value of light-receiving signals from the remaining pixels; and a servo circuit for driving a moving unit so that the total value of the light-receiving signals of the first light-receiving element array obtained by the light-receiving signal operator becomes equal to the total value of the light-receiving signals of the second light-receiving element array obtained by the light-receiving signal operator, the driving of the moving unit bringing the focal point of the objective lens into coincidence with the measuring face.
According to such an arrangement, the convergent light illuminated on the measuring face by the objective lens is specularly and diffusely reflected on the measuring face. The specularly-reflected light and the diffusely-reflected light pass through the objective lens and the like to be subsequently received by the pixels in the first light-receiving element array and the second light-receiving element array.
Then, among the light-receiving signals from the plurality of pixels within the area predetermined in each of the first light-receiving element array and the second light-receiving element array, the light-receiving signals from the pixels of the highest brightness to the nth highest brightness (n being an integer) are excepted for focusing. In other words, focusing is performed based on the darker diffusely-reflected light with exclusion of the specularly-reflected light from the measuring face (i.e., with the brightest pixel being masked). Thus, focusing can be correctly performed. Accordingly, it is possible to prevent a bulging and a recess that are not present in the profile of the measuring face from being generated by false focusing.
However, since the optical displacement measuring instrument according to the document 2 performs the above processings with use of CCD (solid-state image sensor) as the light-receiving element arrays, improvements in the following points are envisaged at the time of practically applying the optical displacement measuring instrument:    (a) due to speckle, diffraction and the like originated from a material and surface irregularities of the target object, the pixels to be masked (i.e. the pixels having the highest brightness to the nth highest brightness among the light-receiving signals) may be discontinuously located, thereby affecting the measurement accuracy;    (b) a complex digital circuit is required for selecting the pixels to be masked (i.e., the pixels having the highest brightness to the nth highest brightness among the light-receiving signals); and    (c) when a measuring instrument that uses doughnut-shaped beam is used and the measuring face is slanted at an angle smaller than the limit angle, the majority of the beam may be masked, so that the applicability may be restricted.